I watched this on ITV3 (UK), last night. I saw a side view of her naked, lying on the ground, and a close, waist-high shot (overhead). Maybe the full frontal was a casualty of a re-think about child nudity on TV. If anything, she looked younger than twelve.

While playing doctor in the woods she nonchalantly strips all the way and lies down in the grass naked, this is more than her male friend was prepared for. He gets embarrassed and runs away. full frontal nudity, very clear.

IMDB lists this movie as "Cider with Rosie (1998) (TV)". They list Perdita's birthday as 25 December, 1985. So I assume this is not a similar title that is listed in 1975. Perdita is not listed in the credits, but this would make her 12 years old and nude on Brazilian TV!